Three phase bidirectional ac-dc converter with bipolar voltage fed resonant stages

ABSTRACT

A bidirectional AC power converter, having a front-end comprising parallel sets of three switches in series, which connects multi-phase AC to coupling transformer through a first set of tank circuits, for synchronously bidirectionally converting electrical power between the multi-phase AC and a DC potential, and for converting electrical power between the DC potential to a bipolar electrical signal at a switching frequency, controlled such that two of each parallel set of three switches in series are soft-switched and the other switch is semi-soft switched; the coupling transformer being configured to pass the bipolar electrical power at the switching frequency through a second set of the tank circuits to a synchronous converter, which in turn transfers the electrical power to a secondary system at a frequency different from the switching frequency.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a Continuation of U.S. patent applicationSer. No. 17/079,396, filed Oct. 23, 2020, now U.S. Pat. No. 11,418,125,issued Aug. 16, 2022, which is a Non-provisional of, and claims benefitof priority from U.S. Provisional Patent Application No. 62/926,206,filed Oct. 25, 2019, the entirety of which is expressly incorporatedherein by reference.

FIELD OF INVENTION

The present invention relates to the field of multiphase integratedbidirectional power electronic converters, and more particularly to sucha converter interfaced with a battery connected to a three-phase gridsystem.

BACKGROUND OF THE INVENTION

In a conventional two-stage AC-DC resonant converter, a three-phaseactive front-end (AFE) boost PFC rectifier feeds an interleaved threephase resonant converter. The input is a low frequency AC power signal,e.g., 50-400 Hz. The resonant converter part contains a high frequencyAC converter followed by a rectifier stage. In the traditional approach,the AC-AC part of the converter requires twelve semiconductors, out ofwhich six are hard-switched, limiting the switching frequency and hencereducing the power density.

Three phase AC-DC converters play a crucial role when the electric poweris consumed in the DC form. With the advent of Energy Storage (ES)applications, Electric Vehicles (EVs) and More Electric Aircrafts (MEA),the demand for high power density (>30 W/in³), high efficiency (>95%)for compact AC-DC converters with the power rating more than 20 kW isincreasing [1-4].

In this regard, a variety of converters have been proposed coveringdifferent aspects [6-9]. In a conventional three-phase interleavedfull-bridge output rectifier configuration as shown in FIG. 1 , 18switches are required [4-5], and in a conventional three, single phaseparallel full-bridge output rectifier configuration as shown in FIG. 2 ,24 switches are required. In these designs, the input power is firstsynchronously rectified using a three-phase interleaved full-bridge (twoswitches per phase) configuration, and then the rectified DC voltage ismodulated as a high frequency AC intermediate signal, using aninterleaved full-bridge (two switches per phase), resulting in arequirement for twelve switches in the front end, half arehard-switched, operating at a limited switching frequency, and half aresoft switched operating at much higher switching frequency.

Hard switching occurs when there is an overlap between voltage andcurrent when switching the transistor on and off. This overlap causesenergy losses which can be minimized by increasing the di/dt and dv/dt.However, higher di/dt or dv/dt causes EMI to be generated. Therefore,the di/dt and dv/dt should be optimized to avoid EMI issues. To minimizethe EMI effects and to improve efficiency, an improved hard switchingtechnique called quasi-resonant switching was developed (mainly seen inflyback converters). In this mode, the transistor is turned on when thevoltage across drain and source is at a minimum (in a valley) in orderto minimize the switching losses and to improve efficiency. Switchingthe transistor when the voltage is at a minimum helps reduce the hardswitching effect which causes EMI. Switching when a valley is detected,rather than at a fixed frequency, introduces frequency jitter. This hasthe benefit of spreading the RF emissions and reducing EMI overall.

Soft switching begins when one electrical parameter reaches zero(current or voltage) before the switch is turned on or off. This hasbenefits in terms of losses. Also, since the switching loss pertransition decreases, the semiconductors can be switched at higherfrequency reducing the size of converter. The smooth resonant switchingwaveforms also minimize EMI. Common topologies like phase-shifted ZVSand LLC are soft switched only at turn-on. For zero voltage switching(ZVS), the transistor will be turned on at zero VDS voltage to reducethe turn-on switching loss. For zero current switching (ZCS), thetransistor will be turned off at zero I_(D) current to reduce the turnoff switching loss.

Most resonant circuits are half- or full-bridge topologies (two or fourtransistors). As transistors are switched on and off, energy can be leftin the transistor and this can cause failure. Due to switching times, ifthis only happens occasionally a rugged body diode is sufficient. If dueto fast transition times it happens continually, then a fast body diodeis required to make sure all the energy will leave the transistor.

A nine-switch power converter design is known [12]. This design is areduced switch topology of conventional twelve-switch back to backconverter. It has three legs with three switches in each of the legcompared to six legs with two switches in each leg of the conventionaltwelve-switch converter. The top switches in each leg along withcorresponding middle switches work as the rectifier and the bottomswitches along with the middle switches work as the inverter. Hence themiddle switches are shared by both rectifier and inverter, reducing theswitch count by 25%. This converter can operate in both constantfrequency mode, where the output frequency is same as the input utilityfrequency and the variable frequency mode, where the output frequency isadjustable.

REFERENCES

(Each of the following is expressly incorporated herein by reference inits entirety)

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SUMMARY OF THE INVENTION

In a conventional two-stage AC-DC p converter, a three-phase activefront-end (AFE) boost power factor correction (PFC) rectifier feeds aninterleaved three-phase resonant converter, as shown in FIGS. 1 and 2 .The resonant converter part contains a high frequency AC converterfollowed by a rectifier stage.

The present invention integrates the front-end boost PFC rectifier andthe high frequency converter stage of the resonant converter into asingle stage low frequency AC to high frequency AC front-end converter.This modified front end feeds the resonant tank and the rectifier stageforming the complete AC-DC converter.

In the traditional approach the AC-AC part of the converter requires 12semiconductors out of which six are hard switched, limiting theswitching frequency and hence reducing the power density. In the presentapproach the same is achieved with nine semiconductors, reducing thecost and increasing the power density. Out of the nine switches, six arecompletely soft switched and the remaining three are semi-soft switched,resulting in much higher switching frequency operation. This makes theconverter solution compact, since the higher switching frequency permitsuse of physically smaller components.

Along with the proposed 9-switch front end converter, based on the typeof rectifier used on the DC side the disclosure proposes two convertervariations. The first variation uses a six-switch output rectifier andis suitable for low power applications (<5 kW). This solution reducesthe total number of semiconductors, without substantially compromisingthe efficiency. The second variation uses twelve switch rectifiers(four-switch full bridge rectifier in each phase) and is suitable forhigh power applications (>5 kW). This solution results in the higherefficiency. Each variation saves three semiconductors in the front-endstage as compared to the prior design, and results in superiorefficiency compared to the conventional solution.

The input ports of the resonant tanks are connected in delta form at thenine-switch front end. The interleaved three phase modulation results ina bipolar voltage input to the resonant tank, and hence the seriescapacitors in the resonant tank are relieved from blocking the DCvoltage. This reduces the stress on these capacitors and increases thestability and the lifetime of the converter. Moreover, the interleavedmodulation results in 120-degree phase between the inputs of eachresonant tank and hence reduces the DC ripple and the filter size on therectifier side.

The present invention thus proposes two related topologies of athree-phase AC-DC isolated converter, based on a front end nine-switchconverter and two different rectifier topologies as shown in FIG. 3(three-phase interleaved full bridge) and FIG. 4 (three, single phaseparallel full bridge).

In a conventional configuration, the three-phase interleaved full bridgeimplementation would require eighteen switches, whereas the presentdesign only needs fifteen semiconductors, resulting in compact converterwith less cost. The conventional three, single phase parallel fullbridge implementation would require twenty-four semiconductors, whereasthe present design needs twenty-one switches. Hence, the converterreduces the device count, and employs more compact cooling, reducingboth the size and cost.

Moreover, the switches in present converter are soft switched allowinghigher intermediate AC switching frequencies, and hence can use lowvolume magnetic components and capacitors, as compared to configurationsthat employ lower intermediate AC switching frequencies, which requirephysically larger magnetic components (inductors, transformers) andcapacitors. Further, the configuration reduces the stress on thecapacitors enabling the stability and long life for the converter.

The present converter integrates a front-end boost PFC rectifier and ahigh frequency converter stage of a typical resonant converter, into asingle stage low frequency AC to high frequency AC front-end converter(the nine-switch front-end converter).

In the present approach, the AC-AC part of the converter is achievedwith nine semiconductors (three less than the twelve semiconductordesigns), reducing the cost and increasing the power density. Out of thenine switches, six are completely soft switched and the remaining threeare semi-soft switched, resulting in much higher switching frequencyoperation. This higher frequency of operation permits the converter tobe compact. This low frequency AC to high frequency AC front-end stageis referred to hereinafter as the nine-switch front-end converter.

Based on the type of rectifier used on the DC side, two convertervariations are provided. The first variation uses a six-switch rectifier(two-switch three-phase interleaved full bridge rectifier in eachphase), and is suitable for low power applications. This solutionreduces the total semiconductors without substantially compromising onthe efficiency. The second variation uses a twelve-switch rectifier(four-switch full parallel bridge rectifier for each phase) and issuitable for high power applications. This solution results in thehigher efficiency. Both the variations save three semiconductors in thefront-end stage with respect to a prior design, and also providesuperior efficiency compared to the conventional solution.

It is noted that the architecture employs conventional three-phasepower, and the design may be readily scaled to accommodate a largernumber of phases in less conventional system architectures. Each phasehas an input structure comprising three switches (MOSFETs) in series,between a positive and negative rectified output, the input of eachphase being between one of the intermediate nodes between the threeswitches, which is typically filtered with a capacitor. The output isdrawn from the other nodes between the switches. The switches in seriesare driven to synchronously rectify the input AC frequency (two softswitched, and one is semi-soft switched), and produce the output at thehigh AC frequency. The high frequency is passed through a tank circuitand transformer to a set of bridges run at the high AC frequency, whichare either three-phase interleaved full bridges or triple single phaseparallel full bridges.

The integrated front end reduces cost compared to conventionalconverters.

The interleaved modulation reduces the DC ripple and the filter size onthe rectifier side, which results in reduction in size of the converter.

The architecture is useful for implementing a medium power AC to DCbidirectional converter for on board electrical vehicle (EV) Charging,energy storage applications, more electric aircraft, and medium voltageelectrical drives, for example.

The technology provides superior performance and efficiency facilitatedby feeding a bipolar voltage to resonant tank circuits. The highfrequency intermediate power transfer helps achieve higher power densityand efficiency, and lower cost. Further, the absence of a sustained DCvoltage in resonant capacitors leads to stable operation and longerlifetime of the converter. The soft-switching of the semiconductordevices enables high switching frequency high efficiency operation.

A typical design with 240 VAC 60 Hz power may be implemented using 1200Vrated, SiC MOSFETs. These switches permit achievement of high powerdensity, high efficiency and provide on-board galvanic isolation. Theintegrated front end reduces cost compared to conventional converters.

The circuits also do a good job of blocking the injection of highfrequency noise back onto the power grid (low THD; high quality power),and reducing output ripple by doing phase balancing according to loadusing interleaved operation.

In accordance with one aspect of the invention, an integrated threephase bidirectional AC to DC converter is provided that interfaces thepower grid with a battery system, and permits battery charging from thegrid, and powering of the grid from the stored energy.

In one embodiment, the present invention provides a nine-switchfront-end converter which is a multiport converter with two three-phaseterminals and a DC link. The nine switches are configured as three setsof three switches in parallel, with the common nodes of the three chainsdriven to a DC voltage.

One set of terminals of the nine-switch front-end converter (e.g., therespective nodes below the high switch) is connected to the grid,whereas the other set of terminals (e.g., the respective nodes above thelow switch) are connected through an L-C resonant tank to primary sidesof three High Frequency (HF) transformers, that provide a galvanicisolation for the converter system. Since the design is bidirectional,power may flow from the first set of terminals to the second set ofterminals, or from the second set of terminals to the first set ofterminals.

The secondary side of the HF transformers are typically connectedthrough another L-C resonant tank to an output side converter, connectedto a battery or DC load. Note that the output side converter need not bea simple AC to DC rectifier, and may produce arbitrary output waveformsdepending on how the switches are driven, and availability of additionalcomponents.

This converter structure (with proper design of resonant elements andmodulation techniques) provides soft switching for six switches of thefront end nine switch converter, and all of the switches of the outputside converter, and provides semi-soft switching for the remaining threeswitches of the nine-switch converter. This allows operation of theconverter with a much higher frequency than the typical operatingfrequency for a conventional converter. In effect, this brings about acompact and light-weight converter system.

The design further includes a phase shift control [10-11] of the bridgesbetween nine switch and output side converters for the optimal powerflow from the grid to the battery (charging mode) as well as from thebattery to the grid (discharging mode).

It is therefore an object to provide a bidirectional AC power converter,comprising: a nine-switch front-end comprising three parallel sets ofthree switches in series, which connects a three phase AC system to aset of three tank circuits having a resonant frequency, the nine-switchfront-end being configured to: synchronously bidirectionally convertelectrical power between the three-phase system and a DC potential onopposite ends of the parallel sets of switches in series, and convertelectrical power between the DC potential and the set of three tankcircuits operated at their respective resonant frequency, such that sixof the nine switches are soft-switched and three of the nine switchesare semi-soft switched; a coupling transformer, configured tobidirectionally couple AC electrical power at the switching frequencyfor each of the set of three tank circuits; and a synchronous converter,configured to transfer the coupled AC electrical power between thecoupling transformer and a secondary system at a switching frequencydifferent from the resonant frequency.

It is also an object to provide a power converter, comprising: afront-end interfacing with a multi-phase AC system, comprising, for eachrespective phase, a set of three switches in series; a capacitor inparallel with each of the sets of three switches in series; a resonanttank circuit for each respective phase, connected between two of the setof three switches in series for a respective phase; a synchronousconverter, configured to interface with a secondary power system; and acoupling transformer, configured to couple power from the resonant tankcircuit for each respective phase to the synchronous converter.

Each of the three switches in series may comprise a MOSFET switch, or aMOSFET switch in parallel with a diode. The MOSFET switches may be SiCMOSFET, e.g., having a voltage rating of >1200V.

The three phase AC system may operates between 30-400 Hz, preferablybetween 50 and 60 Hz. The three phase AC system may operate between 50and 500 VAC, between 90 and 440 VAC, and preferably between 120-240 VAC.The three phase AC system operates, e.g., at 240 VAC between 50-60 Hz.

The tank circuits may each have a resonant frequency between 2 kHz-150kHz.

The switching frequency is e.g., >10 kHz, >25 kHz, >50 kHz, >75 kHz, andmay be, for example, between 50-150 kHz. the resonant frequency of thetank circuits is preferably higher than a three phase AC systemfrequency.

The synchronous converter may comprise a six-switch converter controlledto synchronously convert the AC electrical power at the switchingfrequency, configured as a three-phase interleaved full bridgeconverter. The synchronous converter may alternatively comprise atwelve-switch converter controlled to synchronously convert the ACelectrical power at the switching frequency, configured as threesingle-phase parallel full bridge converters.

Each tank circuit may comprise a capacitor and an inductor, wherein thenine-switch front-end is configured to present a bipolar AC waveform tothe tank circuit that has no DC component.

An automated controller may be provided, configured to control thenine-switch front-end and the synchronous converter.

The automated controller may be configured to perform power factorcorrection, sequence a startup of the bidirectional AC power converter,and/or balance a phase load on the three phase AC system.

The secondary load may be a battery, and the bidirectional AC powerconverter may be configured to charge the battery from the three phaseAC system in a first mode of operation, and to power the three phase ACsystem from the battery in a second mode of operation.

The coupling transformer may comprise a separate primary coil andsecondary coil for each respective phase of the three phase AC system.The coupling transformer may provide galvanic isolation between thethree phase AC system and the secondary load. The coupling transformermay be coupled to the synchronous converter with a respective secondtank circuit for each respective phase.

The synchronous converter may be controlled to produce a dynamicwaveform at the secondary load distinct from a waveform of thebidirectionally coupled AC electrical power at the switching frequency.

A filter capacitor may be provided across the DC potential and/or thesecondary load.

A magnetizing inductance (L_(m)) of the coupling transformer, and thetank circuit may be together configured to maintain zero voltageswitching (ZVS) of at least six switches of the nine-switch front end ata load condition, e.g., load conditions comprising a factor of two orfour.

The bidirectional AC power converter may comprise an automatedcontroller, configured to control the nine-switch front end in a startupmode to: charge the capacitor with a desired DC potential in a rectifiermode of operation; and after charging the capacitor, initiate operationof the tank circuits by switching at a switching frequency of aboutdouble the resonant frequency, and subsequently reduce the frequency ofoperation until a desired output is achieved at the secondary load. Theautomated controller may control the nine-switch front end in alow-power mode below 20% of rated output, to operate the set of threetank circuits in a burst mode of operation wherein the switches arealternately turned on and off for intervals of several switching cycles.The automated controller may control the nine-switch front end in ahigh-power mode above 20% of rated output, to operate the set of threetank circuits in a continuous mode of operation wherein the switches areoperated regularly for each switching cycle. The automated controllermay regulate output power by a phase shift control of the nine-switchfront-end and the synchronous converter.

The power converter may further comprise at automated control,configured to: control the front-end to synchronously convert electricalpower between the multi-phase AC system and a DC potential on thecapacitor, and convert the DC potential on the capacitor into a switchedfrequency which passes through the resonant tank circuits, such that twoof the set of three switches are soft-switched and one of set of threeswitches is semi-soft switched.

The synchronous converter may comprise two switches per phase,configured as a phase interleaved full bridge converter, or fourswitches per phase, configured as a parallel full bridge converter foreach phase.

Each resonant tank circuit may comprise a capacitor and an inductor. Theresonant frequency of the resonant tank circuits may be higher than anoperating frequency of the multi-phase AC system.

An automated controller may control the power converter to perform powerfactor correction, sequence a startup of the power converter, balance aphase load on the multi-phase AC system, and/or regulate output power bya phase shift control of the sets of three switches in series and thesynchronous converter.

The secondary power system may comprise a battery, and the powerconverter may be configured to charge the battery from the multi-phaseAC system in a first mode of operation, and to power the multi-phase ACsystem from the battery in a second mode of operation.

The coupling transformer may be coupled to the synchronous converterthrough a respective second resonant tank circuit for each respectivephase. The synchronous converter may be controlled to produce a dynamicwaveform at the secondary power system distinct from a waveform coupledthrough the coupling transformer.

A magnetizing inductance (L_(m)) of a respective phase of the couplingtransformer, and the resonant tank circuit may be together configured tomaintain zero voltage switching (ZVS) in at least two the set of threeswitches in series at a load condition, e.g., over a range of loadconditions comprising a factor of two or four.

An automated controller may be provided to control the sets of threeswitches in series in a startup mode to charge the capacitor with adesired DC voltage in a rectifier mode of operation; and after chargingthe capacitor, initiate operation of the resonant tank circuits byswitching at a switching frequency of about double a resonant frequency,and subsequently reduce the frequency of operation until a desiredoutput is achieved at the secondary load. The Sets of three switches inseries may be controlled in in a low-power mode below 20% of ratedoutput, to operate the set of resonant tank circuits in a burst mode ofoperation wherein the switches are alternately turned on and off forintervals of several switching cycles. The sets of three switches inseries may be controlled in a high-power mode above 20% of rated output,to operate the set of three tank circuits in a continuous mode ofoperation wherein the switches are operated regularly for each switchingcycle.

It is another object to provide a method of power conversion,comprising: providing a power converter, comprising a front-endinterfacing with a multi-phase AC system, comprising, for eachrespective phase, a set of three switches in series, a capacitor inparallel with each of the sets of three switches in series, a resonanttank circuit for each respective phase, connected between two of the setof three switches in series for a respective phase, a synchronousconverter, configured to interface with a secondary power system, and acoupling transformer, configured to couple power from the resonant tankcircuit for each respective phase to the synchronous converter; andautomatically controlling the set of three switches in series and thesynchronous converter, to control the front-end to synchronously convertelectrical power between the multi-phase AC system and a DC potential onthe capacitor, and convert the DC potential on the capacitor into aswitched frequency which passes through the resonant tank circuits, suchthat two of the set of three switches are soft-switched and one of setof three switches is semi-soft switched. The synchronous converter maybe controlled to operate at a switching frequency different from aresonant frequency of the resonant tank circuit. A resonant frequency ofthe resonant tank circuits may be higher than an operating frequency ofthe multi-phase AC system.

The method may further comprise maintaining zero voltage switching (ZVS)in at least two the set of three switches in series at a load condition.

In a startup mode, the capacitor may be charged with a desired DCvoltage in a rectifier mode of operation, and after charging thecapacitor, operation of the resonant tank circuits initiated byswitching at a switching frequency of about double a resonant frequency,and subsequently reducing the frequency of operation until a desiredoutput is achieved at the secondary load.

The set of three switches in series for each phase may comprise a pairof set of three switches in series, providing a complementary interfacefor each respective phase.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows a prior art (conventional) three phase bidirectional frontend, with three-phase interleaved full bridge output side converters.

FIG. 2 shows a prior art (conventional) three phase bidirectional frontend, with triple single phase parallel full bridge output sideconverters for the respective phases.

FIG. 3 shows a three-phase integrated bidirectional nine-switch frontend, with three phase interleaved full bridge output side converters,according to the present invention.

FIG. 4 shows a three-phase integrated bidirectional nine-switch frontend with triple single phase parallel full bridge output sideconverters, according to the present invention.

FIG. 5 shows a variant of the circuit according to FIG. 4 , with dualnine-switch front ends and differential input connection at input sideof resonant tank, and triple single phase parallel full bridge outputside converters.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 3 and FIG. 4 show the two alternate topologies. In FIG. 3 , thefront-end side converter has three legs. Each leg has three switchesconnected as top, middle and bottom. The midpoints of top and middleswitches of the converter are connected to the three-phase grid, with aninterfacing inductor in each leg.

The midpoints of middle and bottom switches of the converter areconnected to the primary side of three High Frequency (HF) Transformersthrough an L-C resonant link to provide galvanic isolation. The outputside converter has three legs with two switches in each leg. The oneterminal of secondary side of the three HF transformers are connected toeach leg of the output side converter through another L-C resonant link.The other terminal of the secondary side of the three HF transformersare connected to the adjacent leg of the output side converter (i.e. topints ‘b’, ‘c’ and ‘a’ respectively).

An electrolytic DC capacitor is connected at the DC link of thenine-switch front end converter. Another DC capacitor is connected inparallel to the battery at the output side converter to filter out theDC ripple.

In FIG. 4 , the 9-switch front end converter is same as in FIG. 3 .However, the output side converter constitutes three separatesub-converters connected in parallel (each converter with two legshaving two switches in each leg). The two terminals of secondary side ofthe three HF transformers are connected to each leg of thesub-converters through an L-C resonant link. A DC electrolytic capacitoris connected across output of each sub-converter to filter out the DCripple.

FIG. 5 differs in that each input phase is duplicated, and thenine-switch front end is correspondingly duplicated. The midpoints ofmiddle and bottom switches of the legs corresponding to each phase arerespectively connected to a nano-crystalline core based, multi-phasetransformer through an L-C resonant link. The output side is same as inFIG. 4 .

With use of Silicon Carbide (SiC) MOSFETs rated for 1200 V or above,this converter is practically realizable with high switching frequencyoperation (>75 kHz).

To maintain zero voltage switching (ZVS) across different loadingconditions, the design of magnetizing inductance (L_(m)) of the highfrequency transformer, L-C resonant tank design and switch selection areimportant. Therefore, an optimized value of magnetizing inductance(L_(m)) of high frequency transformer and L-C resonant tank design areprovided for the range of loads. A desired switch (SiC MOSFET) based onthe figure of merit (FOM) is selected to have ZVS across differentoutput power levels for both the front end and output side converters.

The switches are operated according to the following control sequence.

Startup Mode:

Charge the DC link capacitor with a desired DC voltage using thenine-switch front-end in a rectifier mode of operation. Thereafter, theL-C resonant converter is turned on with a high frequency (i.e., aroundtwo times the resonant frequency of the converter) and the frequency isreduced until the converter stabilizes to a constant desired output DCvoltage.

Low Power Mode:

At below 20% loads, the L-C resonant converter would be operated inburst mode of operation (the switches will be turned on at certaininterval of few switching cycles).

High Power Mode:

At 20%-100% loads, the L-C resonant converter would be operated withnormal (cycle-continuous) switching operation.

Power Transfer Mode:

Power transfer during both charging and discharging modes for differentoutput power schemes is regulated through a phase shift control of thebridges between the nine-switch front-end and the output sideconverters.

What is claimed is:
 1. An AC power converter, comprising: a front-endcomprising parallel sets of at least three switches in series operatingat a front-end switching pattern with soft or semi-soft switchingexclusively after startup, which connects a multi-phase AC interface toa set of tank circuits having a resonant frequency through a couplingtransformer, and producing a first DC potential; a synchronousconverter, connecting the set of tank circuits to a second DC potential,the synchronous converters operating at a synchronous converterswitching pattern independent of the front-end switching pattern.
 2. TheAC power converter according to claim 1, wherein the AC power converteris bidirectional, and in a first mode transfers power from themulti-phase AC interface to the DC potential, and is a second mode,transfers power from the DC potential to the multi-phase AC interface.3. The AC power converter according to claim 1, wherein multi-phase ACinterface is a three-phase interface, the front end comprises nineswitches arranged as three parallel sets of three switches in series,wherein six switches are soft switched and three switches are semi-softswitched, and the front end is controlled to perform power factorcorrection.
 4. The AC power converter according to claim 3, wherein eachof the switches comprises a MOSFET switch, and wherein the multi-phaseAC system operates between 50 and 500 VAC at between 30 and 400 Hz. 5.The AC power converter according to claim 1, wherein the set of tankcircuits are operated at their resonant frequency of between 2 kHz and150 kHz.
 6. The AC power converter according to claim 1, wherein thefront-end switching pattern comprises a frequency >25 kHz.
 7. The ACpower converter according to claim 1, wherein the synchronous convertercomprises a multi-phase interleaved full bridge converter.
 8. The ACpower converter according to claim 1, wherein the synchronous convertercomprises a plurality of single-phase parallel full bridge converters.9. The AC power converter according to claim 1, wherein each tankcircuit comprises at least one capacitor and at least one inductor. 10.The AC power converter according to claim 1, further comprising anautomated controller, configured to generate front-end switchingpattern, generate the synchronous converter switching pattern, andperform power factor correction.
 11. The AC power converter according toclaim 10, wherein the automated controller is further configured tosequence a startup of the AC power converter according to differentswitching parameters than a normal operating sequence.
 12. The AC powerconverter according to claim 1, wherein the second DC potential isconnected to a battery system, and the AC power converter is configuredin a first operating mode to charge the battery system from themulti-interface, and is configured in a second operating mode to providepower to the AC power interface from the battery system.
 13. The ACpower converter according to claim 1, wherein the synchronous converteris switched to produce a dynamic waveform distinct from a waveformproduced by the front-end.
 14. The AC power converter according to claim1, further comprising an automated controller, configured to control thefront end to: initially in a startup mode, charge a capacitor with thefirst DC potential in a rectifier mode of operation; after charging thecapacitor, initiate operation of the tank circuits, by setting thefront-end switching pattern to a frequency about double the resonantfrequency; and after initiating operation of the tank circuits, reduce afrequency of the front-end switching pattern until a desired output isachieved at the second DC potential.
 15. The AC power converteraccording to claim 1, further comprising an automated controller,configured to selectively control the front end to provide a full powermode of operation wherein the front-end is operated continuously, and alow-power mode of operation wherein the front end is operatedintermittently in a burst mode.
 16. An AC power conversion method,comprising: providing a multi-phase AC interface and a DC interface;providing a front-end comprising parallel sets of at least threeswitches in series, which connect the multi-phase AC interface to a setof tank circuits having a resonant frequency through a couplingtransformer, and producing a front-end DC potential; charging acapacitor with the front-end DC potential in a rectifier mode ofoperation by operating the font-end in a startup switching mode; andafter charging the capacitor initiating operation of the tank circuitsby altering the startup switching mode to an operational switching mode,and synchronously transferring power between the set of tank circuitsand a synchronous converter DC potential at the DC potential interfaceusing a synchronous converter.
 17. The AC power conversion methodaccording to claim 16, further comprising: in a first mode of operation,receiving AC power at the multiphase AC interface, transferring powerfrom the multiphase AC interface through the tank circuits to thesynchronous converter, and delivering DC power through the DC interface;and in a second mode of operation, receiving DC power at the DCinterface, converting the DC power to an AC waveform with a synchronousconverter, transferring the AC waveform through the tank circuits to thefront-end, and delivering multi-phase AC power through the multiphase ACinterface.
 18. The AC power conversion method according to claim 16,wherein multi-phase AC interface is a three-phase interface, the frontend comprises nine switches arranged as three parallel sets of threeswitches in series, wherein six switches are soft switched and threeswitches are semi-soft switched, and the front end is controlled toperform power factor correction.
 19. The AC power conversion methodaccording to claim 15, wherein the startup mode comprises: charge acapacitor with the front-end DC potential in a rectifier mode ofoperation; after charging the capacitor, initiating operation of thetank circuits, by setting a front-end switching pattern to a frequencyabout double the resonant frequency; and after initiating operation ofthe tank circuits, reducing a frequency of the front-end switchingpattern until a desired output is achieved at the DC potentialinterface.
 20. A power converter, comprising: a front-end interfacingwith a multi-phase AC system, comprising, for each respective phase, aset of at least three switches in series, each of the switches beingsoft switched or semi-soft switched in an operation mode; a capacitor inparallel with the sets of three switches in series, storing a front-endDC potential; a resonant tank circuit for each respective phase,connected between two of the set of three switches in series for arespective phase; and a synchronous converter, configured to interfacewith a secondary power system; and wherein the front-end is DC isolatedfrom the synchronous converter.